Labs On Palms: The Story Of Microfluidics

Edited by Navjot Kaur

What happens when irrepressible curiosity meets intractable limitations?

As per the history of science, innovations.

And today we’ll be delving into the story of one such innovation – revolutionary in its concept, radical in its approach, readily adaptable, and endearingly elegant. The story of microfluidics.

But before we bring in the Knight-in-Shining-Armor, a brief word on the damsel in distress.

If you’ve read the previous posts about cell culturing, by now you would’ve developed a reasonably good idea about how we get to see our cells in a regular lab environment. Tiny entities in flat dishes, all at the mercy of over-worked grad students and the occasional nervous intern.

Cell cultures are great – super simple, minimalistic entities. You get to directly perform all your experiments on cells grown in the culture dishes – minimal fuss, minimal effort. But how about a situation wherein you would like to analyze something more complex – say, the interactions of a cell type with its physical environment inside the body? To pile onto this, subtle changes in the physical environment of a cell can cause seismic changes to its behavior. Clearly, the ever-dependable culture plate would come up woefully short. (If you aren’t very familiar with the kind of cell culture setups generally used in labs, do give my earlier piece on this topic a read!)

Well, said the biologists, we’ll go somewhere where we get exactly the right conditions, and (hopefully) the right results. The answer was using animal models – the mouse, the monkey, the zebrafish, and so on. Results came out great, but there was one imposing hurdle – the sheer effort required to maintain them (not to mention ethical issues and protocols). Moreover, working with organisms of such complexity as these, one often had little or no control over numerous intrinsic factors which could send painstakingly designed experiments spinning haywire.

As always throughout the march of history, human ingenuity has swooped in and saved the day. Today, we have advanced models which help accurately replicate the physiological systems – 3D tissue constructs, organoid systems, 3D cell culturing methods, … the works. And to this list of cleverly conceived contraptions, the latter half of the 20th century made one more addition – microfluidic systems. But by no means was this a deus ex machina, furtively smuggled into labs overnight by desperate biologists. Infact, it was the outcome of years of pioneering research, shaped by a plethora of scientific and social forces.  And today, we’re going to take a look at its rich, multi-faceted history.

In essence, microfluidics is about utilizing miniature channels (of the order of micrometers, which are one-millionth of a meter) to flow minute amounts of fluids. The figure here shows what a sample device looks like:

A microfluidic chip I made in the lab (channel design by Dr. Sai Siva Gorthi’s group, Dept. of Instrumentation and Applied Physics, IISc). The channel width is around 250 micrometers (one-fortieth of a centimeter). These are very large channels, by usual microfluidic standards. The transparent slab with the channels running through it is a polymer called PDMS (we’ll be talking about this soon). The circular holes at the ends of each section of the channels are inlets and outlets for tubes which carry solutions – chemicals, cells, etc. The entire setup is mounted on a regular glass slide for support, and easily fits into my pocket.

This, of course was just the original game plan. Why would one want such an equipment? Well, in a large number of experiments, quantity can be an important limiting factor. Sometimes you may have precious few drops of samples to work with. Other times the existing measurement methods would be far too inaccurate for large samples. In any case, large volumes are frequently a major pain for analytical purposes – they’re tough to handle without huge, expensive setups, and easily prone to contamination. Microfluidics solved this problem admirably – whether the sample was scarce or aplenty, all you required were a few pithy drops of solutions to carry out analysis. Starting from simple single reaction measurements, by the 90s, researchers had managed to generate “Total Analysis Systems” wherein a single chip with multiple sub-components could carry out all the steps of a chemical analysis within itself – the first true “Lab-On-A-Chip”.

And while all this was going on in the 70s-90s, a host of seemingly disparate forces were already lining up to unleash a whirlwind of changes.

Towards the end of the Cold War (didn’t see this one coming, did you?), governments began to realize the potential for bio-warfare, as well as the importance of sophisticated, high-performing medical tools for the military. Troops on the field could very well benefit from detectors which could catch biohazard agents, and more importantly, rapid detection of diseases before a major outbreak could help save precious lives. The prime constraints, however, were a far cry from the swanky, polished labs wherein these detection and diagnostic technologies had so far been developed. Out on the field, the Army needed robust, easily portable tools, which needed just basic training to operate, and delivered accurate results with minimum fuss. Microfluidic technology enabled this transition between these starkly different worlds. Tiny chips which could fit into the palm of one’s hand, yet carried all the chemicals and structures needed for sophisticated tests began to be perfected. The DARPA (Defense Advanced Research Projects Agency, USA) poured in an ever-flowing stream of funds which encouraged the academia to pursue cutting-edge research into making microfluidics more dynamic and efficient.

A similar microfluidic chip I had fabricated at Bhushan’s lab (channel design by Dr. Rahul Roy’s group, Dept. of Chemical Engineering, IISc). This weighs in at a more modest 100 micrometer channel width.

The other major influence which gave the applications of microfluidics an even broader context, was the molecular biology revolution.

If the early 20th century could be called the Age of Physics, especially quantum physics, then, the early 21th century would surely be termed the Age of the Life Sciences. And the most influential fore-runner to this was the molecular biology revolution of the late 20th century – the genome sequencing, the ubiquitous polymerase chain reaction, cloning, and so on.

The sequencing and amplifying of small amounts of DNA required high-precision instruments which could handle tiny sample sizes and deliver high accuracy results. Time too, was an important factor. All of these constraints worked well in the favor of advocating microfluidics as a strong candidate for becoming the proverbial workhorse of molecular biologists.

By the late 1990s, and early 2000s, microfluidics had already been adopted for a dizzying array of diverse applications ranging from co-culturing different cell types, sensors for biological activity, sorting specific cell-types out a larger population, and developing prototypes of entire organs (e.g. the liver, the heart, the skin) – all on tiny chips barely half the size of one’s palm. Papers were, and are, being published by the thousands, and every year, we are witnessing previously unimaginably innovative usages to chart out and explore previously forbidden waters.  Not just biological studies, microelectronics is another field which has benefitted tremendously from the microfluidics revolution.

A representative setup, similar to the one we use for our experiments. The syringe pump is essentially a machine with a movable piston that can push out liquid from a loaded syringe at a defined rate. The black arrows shown here would be replaced by silicon tubes in the real-life setup. The actual chip we use is just about the size of a small toffee.

Two major innovations deserve special mention (interestingly, both were pioneered by the same scientist, George M. Whitesides, at Harvard University).

The first, debuting in 1998, was the introduction of a quaint, unassuming polymer called PDMS. A polymer based on silicone, it has since been adopted as standard material for fabricating microfluidic chips. Earlier chips used to be made of plastic, glass, etc. PDMS’ greatest assets were its relative flexibility, transparency, cheap cost, and suitability towards cells. Being an inert substance, PDMS does not react with, or influence the cells it comes in contact with –  a perfect material for making the chips. This was an issue with plastics and glass, as the notoriously finicky cells (if you’ve read the earlier post on cell culturing, you would really appreciate this point) could react with the material of the chip itself, and confound the observed results. PDMS is now one of the most widely used fabricating materials for microfluidic studies.

If the first was a structural change aimed at making existing chips better, the second was the progenitor of a whole field of research in itself – paper-based microfluidics. This relied on replacing the traditional silicone/polymer based devices with ones made out of, literally, paper. To be more specific, cellulose/nitrocellulose fibers were the main constituents of these filter paper-like sheets which could now be modelled to make microfluidic devices.  A major advantage these offered was that unlike the traditional devices which needed pumps to force the liquid flow, these simply relied on the passive absorption of liquid by paper (capillary action) in order to function – ideal for field applications and low-resource regions. And the single best use of such an invention was the one which needed it the most – medical diagnostics. Since lugging expensive laboratory setups to remote villages and far-flung outskirts is a rather tough ask, researchers figured it would be a lot easier if you could fit entire labs onto tiny strips of paper and carry those along! Paper-based microfluidics has been a truly disruptive force in the healthcare field. Researchers are now developing complex tests for diseases on handheld paper strips and plastic bits, rather than the traditional clunky test-tubes and gigantic boxes. Science, for once, can truly reach the very grassroots of society, in every hearth and at every street corner.

A sample paper microfluidic device made by my friend Navjot Kaur. The single channel here shows how the solution (green) progressively moves through the channel during the functioning of the chip. This is an extremely simple, straightforward demonstration of how paper microfluidic devices work.

For the longest time, the requirement of a clunky syringe pump-like instruments to initiate and maintain the actual flow was a primary bottleneck in the introduction of microfluidic chips for remote field applications. Paper microfluidics offers an elegant way around this by largely eliminating the need for syringe pumps, since the liquid can readily get absorbed without any external driving force.

A mere 1500-odd words is embarrassingly low to enumerate the admirable advances made in this field. I’ll be putting up a post very shortly, almost entirely dedicated to illustrating some creative and awe-inspiring applications that these wily devices have been put to. Until then, the more enthusiastic of my readers might take a shot at this lovely review of microfluidics by one of its foremost pioneers, George M. Whitesides:

“The origins and the future of microfluidics” – George M. Whitesides; Nature vol. 442, pages 368–373 (2006)

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